1. Field of the Invention
The present invention relates to a 2,7-carbazole-containing conductive polymer represented by formula 1 and an organic photovoltaic device comprising the conductive polymer as a photoelectric conversion material, and more particularly to a conductive polymer having improved hole mobility, prepared by introducing a small amount of carbazole into a polymer comprising one or more aromatic monomers, and to an organic photovoltaic device having improved energy conversion efficiency, which comprises the conductive polymer as a photoelectric conversion material.
2. Description of the Prior Art
Organic thin film solar cells use organic materials as a photoactive layer and have advantages in that they have a thin thickness of several hundred nm or less, use relatively inexpensive materials as the photoactive layer and can be manufactured as flexible devices. Due to such advantages, many studies on the organic thin film solar cells have been conducted.
The photoactive layer is generally made of two materials having different electron affinities, in which one of the photoactive materials is excited by absorption of light to form excitons, and electrons in the lower electron affinity material (donor) move to the higher electron affinity material (acceptor) across the interface between the two materials so that the excitons are separated into holes and electrons. Herein, the distance by which the excitons can move is about 10 nm, even though it varies depending on materials. Thus, if the distance between the position of light absorption and the interface of the two materials having different electron affinities is about 10 nm or less, the separation of excitons into electrons and holes can be achieved with the highest efficiency. For this reason, a bulk heterojunction structure comprising a mixture of donor and acceptor materials is mainly used.
Methods for manufacturing organic solar cells are broadly divided into two methods: a method of manufacturing a thin film by thermally depositing donor and acceptor materials, and a method of manufacturing a thin film using a solution process.
Specifically, in the method employing thermal deposition, both the donor and acceptor materials are monomers, whereas in the method employing the solution process, the donor material is generally a polymer, and the acceptor material is a polymer, a fullerene derivative, a perylene derivative, inorganic quantum dot nanoparticles, or the like. Thus, when the solution process employing polymers is used, large-area devices can be manufactured in an inexpensive manner compared to when monomers are thermally deposited. For this reason, in recent years, studies on the solution process employing polymers have been actively conducted.
Till now, the use of a fullerene derivative as the acceptor material has showed the highest efficiency [J. Am. Chem. Soc., 2008, 130(48), 16144]. In order to increase photoelectric conversion efficiency, tandem-type devices comprising an intermediate electrode sandwiched between two polymers that absorb light in different regions have also been developed [Science, 2007, 317, 222].
Organic solar cells should satisfy high photoelectric conversion efficiency. To achieve high photoelectric conversion efficiency, the following requirements should be satisfied. First, a large amount of photons should be absorbed in the light absorption layer. Second, the absorbed and excited excitons should move to the donor/acceptor interface so that they should be effectively separated into holes and electrons. Third, the separated holes and electrons should move to the positive and negative electrodes without loss.
When a fullerene derivative is used as an acceptor material in the bulk heterojuction structure, the separation of the second excitons appears to occur quantitatively. Thus, in order to achieve high photoelectric conversion efficiency in organic thin film solar cells, a polymer that is used as the donor material should satisfy both the property of absorbing a large amount of photons and the ability to move holes.
Accordingly, the present inventors have made extensive efforts to achieve high photoelectric conversion efficiency in an organic thin film solar cell and, as a result, have devised a molecular design of an electron donor using a carbazole group which is frequently used due to high hole mobility in organic light-emitting diodes (OLEDs) and organic thin film transistors (OTFTs), thereby completing the present invention.